There are already known various constructions of sensing arrangements, among them such capable of sensing stresses within a structure. So, for instance, it is known from the commonly owned U.S. Pat. No. 4,806,012, issued on Feb. 21, 1989 and entitled "Distributed, Spatially Resolving Optical Fiber Strain Gauge", to embed in the structure the stresses of which are to be determined an optical fiber containing a plurality of periodic Bragg gratings of different original periodicities and thus each reflecting light in a narrow range around a central wavelength that is determined by the respective periodicity. These gratings are disposed at different regions of the structure so as to be subjected to different stresses, temperatures and strains depending on their locations in the structure, with attendant strain-related changes in their periodicities and hence in their central wavelengths of reflection. During the use of this known sensing arrangement, light is launched into the optical fiber in a such a wavelength range as to embrace the wavelengths of interest with respect to all of the Bragg gratings under all conditions. Then, either the light returned back to the launching end of the fiber is examined for the presence, or that reaching the other end for the absence or diminished intensity, of light around the respective central wavelengths of the gratings as altered by the stresses existing at the respective locations of the structure, thereby to determine the magnitude of such stresses.
The individual Bragg gratings are provided in the optical fiber core, prior to its installation in the structure, by exposing the optical fiber core through the cladding to an interference pattern of two ultraviolet light beams the light frequency and/or orientation of which relative to the optical fiber longitudinal axis is such, for each of the gratings, that the interference pattern maxima and minima extend through the fiber in directions normal to the longitudinal axis and that the periodicity (e.g. the distance between two consecutive maxima) is that desired for the particular grating.
As advantageous as this approach is for use in many applications in which the number of locations along the optical fiber to be monitored for stresses in the structure is relatively small, it has an important drawback in that each of the gratings has to have assigned to it a considerable amount of the available spectrum (i.e. not only its relatively narrow wavelength or frequency band but also the separation from the adjacent wavelength or frequency band assigned to another grating by an amount sufficient to avoid overlapping of or crosstalk between the adjacent channels under all circumstances, that is, even when the central wavelengths of the adjacent channels have moved, as a result of the stresses applied at the locations of the associated gratings, toward one another to the maximum extent), thus severely limiting the number of grating sensors that can be employed within each sensing optical fiber. For example, if a solid state device, such as an edge emitting diode or a laser diode were used as the light source, which would be highly desirable because of the relatively low cost and reliability of such device, it would only cover a wavelength range of 20 to 30 nm. On the other hand, each of the optical strain sensors of the above type would require up to 5 nm of bandwidth to cover 10,000 microstrain, so that only 4 to 6 sensors could be associated with each diode source.
Accordingly, it is a general object of the present invention to avoid the disadvantages of the prior art.
More particularly, it is an object of the present invention to provide a distributed optical fiber Bragg grating sensor arrangement which does not possess the disadvantages of the known arrangements of this kind.
Still another object of the present invention is so to develop the distributed sensor arrangement of the type here under consideration as to significantly increase the number of locations stresses from which data indicative of the quantity being monitored can be reliably collected using a single optical fiber containing a multitude of embedded Bragg gratings.
It is yet another object of the present invention to devise an arrangement of the above type which renders it possible to utilize a relatively inexpensive laser with limited bandwidth as a light source for launching light into the optical fiber for examination of the various Bragg gratings.
A concomitant object of the present invention is to design the sensing arrangement of the above type in such a manner as to be relatively simple in construction, inexpensive to manufacture, easy to use, and yet reliable in operation.